Crude argon having an argon concentration of about 98 percent or less is produced by the cryogenic rectification of air. Argon comprises less than 1 percent of air. Typically air is separated into oxygen and nitrogen by use of a double column system comprising a higher pressure column in heat exchange relation with a lower pressure column. At or near the point in the lower pressure column where the concentration of argon is at a maximum, a stream is withdrawn from the lower pressure column and passed into an argon column for rectification into crude argon. The argon concentration of the argon column feed stream is about 7 to 12 percent so that effective argon recovery can be attained by use of the argon column system. The remainder of the argon column feed stream comprises oxygen and nitrogen.
In the argon column the feed is separated by cryogenic rectification. The less volatile component, oxygen, concentrates at the bottom of the column and the more volatile argon concentrates at the top of the column. Nitrogen, which is even more volatile than argon, goes with the argon.
From the top of the column a crude argon stream, generally comprising about 95 to 98 percent argon is removed for further processing to produce high purity or refined argon. The remainder of the crude argon stream comprises oxygen and nitrogen.
Oxygen is removed from the crude argon stream by mixing it with hydrogen and passing the mixture through a catalytic hydrogenation unit wherein the hydrogen and oxygen react to form water. The stream is then passed through a dryer for the removal of the water. Alternatively the oxygen may be removed from the crude argon stream by kinetic adsorption thereby reducing or eliminating the need for catalytic hydrogenation and the associated hydrogen requirements.
Once the oxygen has been removed from the crude argon stream, the nitrogen is separated from the argon by cryogenic distillation. The resulting high purity or refined argon, having an oxygen concentration generally less than 2 ppm and a nitrogen concentration generally less than 2 ppm, is now suitable for commercial use.
The capital and operating cost of producing refined argon from the crude argon recoverable from the argon column system is considerable and thus it is desirable to have a system whereby refined argon may be recovered directly from the argon column system.
It is known that the separation of argon and oxygen in the argon column may be essentially complete if sufficient equilibrium stages are incorporated into the argon column. Generally at least 150 equilibrium stages in the argon column are required for this purpose. In such a situation essentially all of the oxygen in the argon column feed is separated from the argon and the crude argon removed from the top of the column contains essentially no oxygen. However, because of the relative volatilities of these components, the nitrogen goes with the argon and thus a separate nitrogen removal step is still required to process the crude argon stream into refined argon
Accordingly it is an object of this invention to provide a cryogenic rectification method which enables one to recover nitrogen-free argon directly from an argon column system.
It is another object of this invention to provide a cryogenic rectification method which enables one to recover refined argon directly from an argon column system